Electrodeless fluorescent lamps are disclosed in U.S. Pat. No. 3,500,118 issued Mar. 10, 1970 to Anderson; U.S. Pat. No. 3,987,334 issued Oct. 19, 1976 to Anderson; and Anderson, Illuminating Engineering, April 1969, pages 236-244. An electrodeless, inductively-coupled lamp, as disclosed in these references, includes a low pressure mercury/buffer gas discharge in a discharge tube which forms a continuous, closed electrical path. The path of the discharge tube goes through the center of one or more toroidal ferrite cores such that the discharge tube becomes the secondary of a transformer. Power is coupled to the discharge by applying a sinusoidal voltage to a few turns of wire wound around the toroidal core that encircles the discharge tube. A current through the primary winding creates a time-varying magnetic flux which induces along the discharge tube a voltage that maintains the discharge. The inner surface of the discharge tube is coated with a phosphor which emits visible light when irradiated by photons emitted by the excited mercury atoms. The lamp parameters described by Anderson produce a lamp which has a high core loss and is therefore extremely inefficient. In addition, the Anderson lamp is impractically heavy because of the ferrite material used in the transformer core.
An electrodeless lamp assembly having high efficiency is disclosed in U.S. application Ser. No. 08/624,043, filed Mar. 27, 1996 now U.S. Pat. No. 5,834,905. The disclosed lamp assembly comprises an electrodeless lamp including a closed-loop, tubular lamp envelope enclosing mercury vapor and a buffer gas at a pressure less than about 0.5 torr, a transformer core disposed around the lamp envelope, an input winding disposed on the transformer core and a radio frequency power source coupled to the input winding. The radio frequency power source typically has a frequency in a range of about 100 kHz to about 400 kHz. The radio frequency source supplies sufficient radio frequency energy to the mercury vapor and the buffer gas to produce in the lamp envelope a discharge having a discharge current equal to or greater than about 2 amperes. The disclosed lamp assembly achieves relatively high lumen output, high efficacy and high axial lumen density simultaneously, thus making it an attractive alternative to conventional VHO fluorescent lamps and high intensity, high pressure discharge lamps.
When a discharge is sustained by high frequency alternating current, magnetic interference can be a serious problem. The problem increases in severity as the magnitude of the current and the dimensions of the discharge increase. In the case of an inductively coupled electrodeless light source, this is a practical problem that must be addressed to satisfy electromagnetic interference (EMI) regulations.
The source of near field magnetic interference in an electrodeless light source is the magnetic field created by the high frequency discharge current. Prior art methods of magnetic field suppression are based on placing a closed conductive ring or loop next to the discharge, such that the discharge current induces a counter current in the loop. In principle, the counter current creates a magnetic field of proper magnitude and phase to counteract the magnetic field of the discharge. This arrangement of a counter current loop positioned near a discharge current loop generally results in loose coupling. However, loose coupling cannot provide good balance between the discharge current and the loop current. Thus, these prior art techniques result in rather limited magnetic field suppression.
U.S. Pat. No. 4,409,521 issued Oct. 11, 1983 to Roberts discloses a circular electroded fluorescent lamp having a cancellation loop external to the lamp envelope, which produces a magnetic field generally in opposition to the magnetic field generated by the current in the arc discharge.
U.S. Pat. No. 5,539,283 issued Jul. 23, 1996 to Piejak et al discloses a discharge light source having reduced magnetic interference. A conductive loop surrounding a driving inductor is terminated in a capacitor to lower the impedance of the compensation circuit, so that the compensation current is increased.
U.S. Pat. Nos. 4,568,859; 4,645,967; 4,704,562; 4,727,294; 4,920,297 and 4,940,923 disclose a set of conductive short circuited rings that surround a lamp envelope. When a discharge is inductively excited, the rings create a current which induces a magnetic flux, in a direction opposite to the primary flux, that neutralizes some of the magnetic flux of the primary induction coil. This technique is not very effective and is found to reduce the magnetic flux emitted from the discharge by only about 2.0 dB per ring.
Additional techniques for suppressing electromagnetic interference in electrodeless discharge lamps are disclosed in U.S. Pat. No. 4,245,179 issued Jan. 13, 1981 to Buhrer; U.S. Pat. No. 4,254,363 issued Mar. 3, 1981 to Walsh; and U.S. Pat. No. 5,239,238 issued Aug. 24, 1993 to Bergervoet et al. The disclosed techniques are understood to lack effectiveness and practicality.
Accordingly, improved techniques for reducing electromagnetic interference generated by electrodeless discharge lamps are required.